Opto-electronic ignition systems for internal combustion engines

ABSTRACT

An opto-electronic ignition system for an internal combustion engine in which radiation falls on to and is cut off from a photo-transistor in timed sequence with the engine, and in which between the photo-transistor and the primary winding of the ignition coil there is a plurality of switching Darlington pairs, each Darlington switching in inverse relation to its neighbor, and the first Darlington switching in inverse relation with the photo-transistor. The circuit is designed so that it will reliably operate in engine compartment temperatures above 125° C.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of Ser. No. 655,138, nowabandoned, filed Feb. 3, 1976.

.Iadd.Applicant hereby claims the priority date of British Patent Appln.No. 6028/75 filed Feb. 12, 1975. .Iaddend.

FIELD OF THE INVENTION

The present invention relates to opto-electronic ignition systems suchas disclosed in my U.S. Pat. Nos. 3,605,712 and 3,710,131.

DESCRIPTION OF THE PRIOR ART

In these prior patents the transistors which form the bistable triggerare arranged to switch in inverse relation to one another, so that ifany one transistor is fully saturated its neighbors are non-conductive.The purpose of this type of bistable trigger is to ensure that at anyinstant of time there is always one transistor fully conductive to actas a short-circuit path for any transients on the line.

In my U.S. Pat. No. 3,710,131, three inverse switching transistors areemployed between the output of the photo-transistor 2 and the powertransistor P. It has also been proposed to use a power Darlington pairin place of the single power transistor. Furthermore, it is known frommy U.S. Pat. No. 3,682,150 to replace the photo-transistor by aDarlington pair, one of said components being a photo-transistor. Theuse of Darlington pairs in ignition circuits is also known from BritishPat. No. 1,314,052 (Bowen) in which a Darlington pair (TR1 and TR2) anda power Darlington pair (TR3 and TR4) switch "on" and "off" together.

The circuit arrangement proposed in British Pat. No. 1,314,052 isextremely disadvantageous firstly because of the slow switching due tothe fact that the power Darlington pair can only be switched "on" or"off" through the first Darlington pair, and secondly because there isno transient protection for the Darlington pairs which means that thepower Darlington pair in particular is extremely vulnerable totransients.

Three stages of inverse switching between the photo-transistor and thepower transistor are used in the system disclosed in my U.S. Pat. No.3,710,131 in order to amplify the low current of about 1 mA through thephoto-transistor up to a current of about 10 Amps adequate for thedesired operation of the primary winding of the ignition coil.

A further problem arises in connection with the photo-transistor at highambient temperatures. Under extreme conditions, the temperature withinthe engine compartment may reach 150° C. It has been found that attemperatures of above about 125° C., the leakage current through thephoto-transistor, when in the "off" state, rises very sharply indeed.

Experiments have shown that in fact two different conditions arise whichcan result in the failure of previous inverse switching circuits tooperate at extremely high temperatures, e.g., above 125° C. NPNtransistors typically exhibit the following characteristics:

(a) V_(CE) sat increases with temperature.

(b) V_(BE) decreases with temperature.

Thus, inverse switching circuits using ordinary NPN transistors arevulnerable to two modes of failure:

(a) The value of V_(CE) sat of the photo-transistor rises withincreasing temperature until the first transistor of the chain ofinverse switching transistors turns "on" instead of being held "off,"i.e., when V_(CE) sat of photo-transistor exceeds V_(BE) of the firsttransistor.

(b) The leakage current through the photo-transistor rises exponentiallywith temperature. This requires the use of a high gain device for thefirst stage of inverse switching, i.e., a device which will remain "on"at high temperature in spite of losing its base drive which is divertedthrough the photo-transistor.

In connection with the circuit disclosed in my U.S. Pat. No. 3,682,150,it should be mentioned that I have found the use of a photo-Darlingtonpair actually amplifies the leakage problem and also has the undesirableeffect of substantially reducing the speed of switching.

Also, in the Bowen circuit due to the fact that all semiconductordevices when "off" have leakage currents which increase exponentiallywith temperature, the presence of any substantial leakage currentthrough the first Darlington pair will be fed direct to the output stageconsisting of the second Darlington pair. Due to the high gain ofDarlington devices, it would not need much leakage current through thefirst transistor of the first Darlington pair to turn on the outputpower Darlington stage and there is accordingly a real danger that thiscould happen at temperatures above 125° C.

SUMMARY OF THE INVENTION

A principal object of the invention is to provide an opto-electronicignition system which, although capable of providing rapid switching ofthe ignition coil, includes a minimum number of switching stages.

Another important object of the invention is to provide such anopto-electronic ignition system which is capable of reliable operationwhen subjected to elevated engine compartment temperatures.

According to the present invention, there is provided an opto-electronicignition system for controlling switching of the primary winding of anignition coil of an internal combustion engine and capable of reliableoperation at temperatures above about 125° C., said system comprising asource of radiation; a photo-transistor sensitive to radiation whichwill switch on or conduct when exposed to the radiation and switch offwhen the radiation is cut off; means disposed between said source andsaid photo-transistor for intermittently blocking radiation from saidsource in timed relation to the engine revolutions so as to control theperiods during which radiation is received by said photo-transistor; anda switching circuit for controlling switching of the ignition coilresponsive to the output of said photo-transistor, said switchingcircuit comprising a plurality of pairs of transistors the components ofeach pair being connected in a Darlington configuration and means forconnecting said pairs of transistors to each other between saidphoto-transistor and the ignition coil such that each Darlington pairswitches in inverse relationship to at least one other Darlington pairand a first Darlington pair switches in inverse relationship to saidphoto-transistor, so as to cause fast switching of the primary windingof the ignition coil thereby inducing a spark voltage in the secondwinding of the coil.

It has been found that the use of a Darlington pair as the first stagein a chain of inverse switching transistors is capable of eliminatingthe photo-transistor leakage problem referred to above without affectingthe operation of the circuit or the speed of switching.

A parallel circuit consisting of a capacitor and at least one zenerdiode, and a resistor connected in series therewith, may be connectedbetween the commoned collector electrodes of the first transistor of thefinal Darlington stage, to thereby protect this stage against positivegoing transients, and control the rate at which the final stage switchesoff, thereby eliminating the generation of radio interference.

A zener diode may be provided to supply a stabilized voltage to thesource of radiation, the photo-transistor and the first Darlingtonstage.

A first diode may be connected across the emitter-collector path of thephoto-transistor, and a second diode may be connected across the sourceof radiation, these diodes serving to protect these components againstreverse battery connection.

A diode may be connected between each junction of the Darlington pairsof the circuit, and the commoned emitter electrodes of all the secondtransistors of the Darlington pairs, to protect the circuit againstnegative going transients and reverse battery connection.

The Darlington pairs, with the possible exception of the last in thecircuit, which may be a power Darlington pair, may be formed on amonolithic chip, together with the photo-transistor.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will now be described in greater detail, by way ofexample, with reference to the accompanying drawing, wherein the soleFIGURE is a circuit diagram of one preferred form of an opto-electronicignition system.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawing, the circuit includes a solid state galliumarsenide infra-red lamp 1, a photo-transistor 2 and two pairs ofDarlington pairs Q1 and Q2. The lamp 1, the photo-transistor 2 and thefirst Darlington pair Q1 are each in series with respective resistors R1and R2, and R4, and receive a 7.5 volt stabilized supply from a zenerdiode Z1. The zener diode Z1 is connected across the 12 volt battery ofthe vehicle through a resistor R3 and thus supplies all three componentsmentioned above with a stabilized voltage supply. The advantage ofadditionally stabilizing the voltage across the first Darlington pair Q1will be explained later on. A diode D5 is connected across the lamp 1 toprotect it against reverse battery connection.

Connected across the emitter-collector electrodes of thephoto-transistor 2 is a diode D1, which serves not only to ensure cleanswitching of the photo-transistor 2, but ensures that any negativetransients on the line during the time that the photo-transistor isnon-conductive are conducted past the photo-transistor and are thusunable to cause any damage to its structure.

The Darlington pairs Q1 and Q2 each consist of a pair of transistors T1,T2 and T3, T4 respectively, arranged in conventional Darlingtonconfiguration. The base electrode of the transistor T1 is connected tothe collector electrode of the photo-transistor 2. The commonedcollector electrodes of the transistor T1 and T2 are connected firstlyto the base electrode of the transistor T3 of the second Darlington pairQ2 through a diode D3, and, secondly, to the 7.5 volt zenered supplythrough a resistor R4.

A diode D2 is connected across the commoned collector electrodes of thetransistors T1 and T2, and the emitter electrode of the transistor T2.The diode D2 serves to protect the Darlington pairs against negativegoing transients, and also against a careless mechanic connecting thebattery the wrong way around. The diode D1 also serves to protect thephoto-transistor 2 against reverse battery connection.

The commoned collector electrodes of the transistors T3 and T4 areconnected to one end of the primary winding of the ignition coil 4, theother end of which is connected to the positive terminal of the 12 voltbattery. Between the commoned collector electrodes of the transistors T3and T4, and the base electrode of the transistor T3, there is connecteda circuit comprising a pair of zener diodes Z2 and Z3 and a resistor R5all connected in series and a capacitor C connected in parallel with thezener diodes Z2 and Z3. The purpose of the circuit is firstly to controlthe rate of switching of the Darlington pair Q2, and secondly, toprotect the Darlington pair against positive going transients. Withoutthis circuit, the voltage induced in the primary winding of the ignitioncoil consists of a first positive going spike which will rise verysharply and in extreme cases will reach a peak of 400 volts followed bya series of smaller spikes. The presence of the zener diodes Z2 and Z3ensures that the Darlington pair Q2 will be turned on again when thispositive going spike exceeds the combined zener voltage of the two zenerdiodes. The capacitor C has the effect of increasing the duration of therise time of the first positive going spike, thus ensuring that anyradiation which would have been generated by an uncontrolled positivegoing spike is eliminated thus preventing radio interference.

A diode D4 is connected between the commoned collector electrodes of thetransistors T3 and T4 and the emitter electrode of the transistor T4 ofthe Darlington pair Q2 and protects the device against reverse batteryconnection.

It is stated above that the zener diode Z1 provides a stabilized 7.5volt supply to the Darlington pair Q1 in addition to the infra-red lamp1 and the photo-transistor 2. Furthermore, it is essential that thediode D3 has a minimum storage time in the base of Darlington pair Q2,and both a minimum forward-recovery time and a minimum reverse recoverytime. This is achieved by the zenered supply to the resistor R4 in thecollector circuit of the Darlington pair Q1. Operated under theseconditions the diode D3 together with the circuit consisting of zenerdiodes Z2 and Z3, the capacitor C and the resistor R5, helps to ensurethat there is no radio interference generated on switching theDarlington pairs.

With regard to the operation of the circuit at temperatures above 125°C., it has been found that the use of the Darlington pair Q1 as thefirst stage of inverse switching instead of a single transistor, reducesthe risk of failure. This is firstly because the value of V_(BE) of thefirst stage is double that of a single transistor. Moreoever, since theDarlington pair Q1 will have a high gain, its operation in the inverseswitching chain of Darlington pairs is unaffected when it loses its basedrive through leakage across the photo-transistor. Accordingly, thecircuit is able to operate satisfactorily when subjected to enginecompartment temperatures above 125° C.

In one preferred form, the Darlington pair Q2 is a power Darlingtonpair, thus ensuring sufficient current carrying capacity to handle thecurrents flowing through the primary winding of the coil when themagnetic field of the coil is being built up.

In the above-described embodiment the photo-transistor 2, and theDarlington pairs Q1 and Q2 all switch in inverse relation to oneanother. Thus, when the photo-transistor 2 receives the infra-redradiation from the solid-state lamp 1 it will switch on in the fullysaturated condition, which means that the Darlington pair Q1 isnon-conductive, whilst the Darlington pair Q2 is fully on, passing theignition coil current through the primary winding of the coil 4. As soonas the infra-red radiation is cut off from the photo-transistor 2, it isrendered non-conductive, which switches the Darlington pair Q1 on, andthe Darlington pair Q2, off. When the Darlington pair Q2 switches offthe primary current is interrupted to cause the collapse of the magneticfield associated with the coil 4, and the induction of a high voltage inthe secondary winding to produce the spark.

It will be appreciated that there it is desired to switch in the reversemode, i.e., the spark is produced when the photo-transistor 2 isrendered conductive, all that is necessary is to introduce a furtherDarlington pair in the chain of Darlington pairs.

The switching on and off of the photo-transistor is achieved by means ofa bladed disc 3, such as disclosed in my U.S. Pat. No. 3,710,131, or anapertured disc such as the type disclosed for the first trigger in myU.S. Pat. No. 3,981,282. Preferably, where the disc is driven from thecam shaft of the engine the number of apertures or slots is equal to thenumber of cylinders in the engine, but if the disc is not mounted withinthe distributor housing, as is conventional, the number of slots orapertures may be a multiple or sub-multiple of the number of cylindersin the engine. A drive for the disc 3 is indicated at 10.

As disclosed in my copending U.S. application Ser. No. 759,061, filedJan. 13, 1977 as a continuation-in-part application on my U.S.application Ser. No. 652,748 now abandoned, the Darlington pair Q1, orin the case of the reverse mode switching trigger, the first twoDarlington pairs in the circuit may be formed on a monolithic chip,together with the photo-transistor 2, the diode D1 and the resistors R2and R4, appropriate interconnections being made by well known integratedcircuit techniques. In this construction the monolithic chip is mountedwithin the distributor housing opposite the gallium arsenide lamp,between which the blades of the chopper disc rotate. The powerDarlington pair is mounted on a separate heat sink outside thedistributor housing, together with the resistors R3 and R5, the zenerdiodes Z1 and Z3, and the diodes D2 and D3.

A further advantage arising out of the use of a Darlington pair as thefirst stage of inverse switching is that because of the high gain of thedevice it is possible to use a smaller photo-area for the base of thephoto-transistor than that proposed in my copending application Ser. No.759,061 referred to above. It is thus possible to match thecharacteristics of the photo-transistor 2 to those of the infra-redgallium arsenide lamp 1 in order to keep the switching point accuratethroughout the range. Although a reduction in size of the photo-areawill mean a reduction in photo-current passed by the photo-transistor,this is more than made up by the gain of the first Darlington pair Q1.

Alternatively, in cases where heat dissipation is not a serious problem,the power Darlington pair may also be formed on the monolithic chip.

Although in the above embodiment there are two series connected zenerdiodes across the Darlington pair Q2, it is possible to provide only asingle zener diode if the voltage rating is high enough to effectsatisfactory operation of the Darlington pair in the presence oftransients.

It will be appreciated that the chain of Darlington pairs can have two,four or six pairs for one mode of switching, or three, five or sevenpairs for the reverse mode of switching.

It will be further appreciated that a diode is inserted between eachDarlington pair and ground.

What I claim and desire to secure by Letters Patent is:
 1. Anopto-electronic ignition system for controlling switching of the primarywinding of an ignition coil of an internal combustion engine and capableof reliable operation at temperatures above about 125° C., said systemcomprising a source of radiation; a photo-transistor sensitive toradiation which will switch on or conduct when exposed to the radiationand switch off when the radiation is cut off; means disposed betweensaid source and said photo-transistor for intermittently blockingradiation from said source in timed relation to the engine revolutionsso as to control the periods during which radiation is received by saidphoto-transistor; and a switching circuit for controlling switching ofthe ignition coil responsive to the output of said photo-transistor;said switching circuit comprising a plurality of pairs of transistorsthe components of each pair being connected in a Darlingtonconfiguration and means for connecting said pairs of transistors to eachother between said photo-transistor and the ignition coil such that eachDarlington pair switches in inverse relationship to at least one otherDarlington pair and a first Darlington pair switches in inverserelationship to said photo-transistor, so as to cause fast switching ofthe primary winding of the ignition coil thereby inducing a sparkvoltage in the second winding of the coil.
 2. An opto-electronicignition system according to claim 1, wherein the radiation source is agallium arsenide lamp emitting radiation in the infra-red region of theelectro-magnetic spectrum.
 3. An opto-electronic ignition systemaccording to claim 1, wherein the last Darlington pair in the circuit isa Darlington power pair, the emitter-collector path of the secondtransistor being connected in series with the primary winding of theignition coil.
 4. An opto-electronic ignition system according to claim3, wherein means are provided for protecting the Darlington powertransistor against transients and for slowing down the fast switch offof said power Darlington pair.
 5. An opto-electronic ignition systemaccording to claim 4, wherein said means includes a parallel circuitconsisting of a capacitor and at least one zener diode, and a resistorconnected in series therewith, said parallel circuit being connectedbetween the commoned collector electrodes and the base electrode of thefirst transistor of the Darlington power pair.
 6. An opto-electronicignition system according to claim 4, wherein a diode is connectedbetween the emitter electrode of the second transistor and the baseelectrode of the first transistor of the Darlington power pair, tothereby protect the Darlington power pair against negative goingtransients.
 7. An opto-electronic ignition system according to claim 2,wherein a zener diode is connected across both the gallium arsenidelamp, the photo-transistor and the first Darlington pair to provide astabilized voltage source therefor.
 8. An opto-electronic ignitionsystem according to claim 2, wherein a first diode is connected acrossthe emitter-collector path of the photo-transistor, and a second diodeis connected across the gallium arsenide lamp.
 9. An opto-electronicignition system according to claim 1, wherein there is an even number ofDarlington pairs in the transistorized ignition circuit, the spark forignition being produced on de-energization of the photo-transistor. 10.An opto-electronic ignition system according to claim 1, wherein thereis an odd number of Darlington pairs in the transistorized ignitioncircuit, the spark for ignition being produced on energization of thephoto-transistor.
 11. An opto-electronic ignition system for controllingswitching of the primary winding of an ignition coil of an internalcombustion engine and capable of reliable operation at temperaturesabove about 125° C., said system comprising an infra-red solid statesource of radiation; a photo-transistor sensitive to infra-red radiationwhich will switch on or conduct when exposed to the radiation and switchoff when the radiation is cut off; means arranged between said infra-redradiation source and said photo-transistor for intermittently blockingradiation from said infra-red source in timed relation to the enginerevolutions so as to control the periods during which the infra-redradiation is received by said photo-transistor; and a switching circuitfor controlling switching of the ignition coil responsive to the outputof said photo-transistor, said switching circuit comprising first andsecond pairs of transistors connected in a Darlington configuration andmeans for connecting said pairs of transistors to each other betweensaid photo-transistor and the ignition coil such that the two Darlingtonpairs switch in inverse relationship to each other and the said firstDarlington pair switches in inverse relationship to saidphoto-transistor so as to cause fast switching of the primary winding ofthe ignition coil thereby inducing a spark voltage in the second windingof the coil.